1. Technical Field of the Invention
The present invention relates to a transmitting circuit for use in, for example, a portable telephone, and a transmitter-receiver provided with the transmitting circuit.
2. Prior Art
FIG. 3 is a structural view showing a conventional transmitter-receiver, a transmitting circuit 31 has a power amplifier 32 and an isolator 33, and a transmitting signal is amplified by the power amplifier 32, and is outputted to an antenna 35 via the isolator 33 and a duplexer 34 sequentially.
On the other hand, a receiving signal received by the antenna 35 is inputted to a receiving circuit 36 via the duplexer 34.
In this case, if impedance between the power amplifier 32 and the antenna 35 is matched, all transmitting signals to be outputted from the power amplifier 32 will be transmitted to the antenna 35. If, however, for example, the antenna 35 is broken and is disconnected from the duplexer 34, the transmitting signals to be transmitted will be total-reflected to be returned to the amplifier 32 side. At this time, without the isolator 33, a reflected wave would be inputted to the power amplifier 32, and current twice stronger at maximum would flow through the power amplifier 32. Therefore, the power amplifier 32 may possibly be damaged by a thermal runaway. In order to prevent it, the isolator 33 is provided in the latter stage of the power amplifier 32.
The isolator effectively functions as means to prevent the power amplifier from being damaged due to a reflected wave, but the first drawback is its high cost. Also, a request for miniaturization of the apparatus has recently been more increased, but the isolator is larger in shape than other general passive components, which is a factor for preventing the miniaturization.
Thus, in a transmitting circuit and a transmitter-receiver according to the present invention, it is an object to prevent the power amplifier from being damaged due to a reflected wave without the aid of any expensive isolator, and to miniaturize.
In order to solve the above-described problems, a transmitting circuit according to the present invention has a power amplifier and a directional coupler provided between the power amplifier and an antenna, for detecting a reflected wave reflected from the antenna side on the power amplifier side in such a manner that when a level of the reflected wave exceeds a predetermined value, supply of power supply voltage applied to the power amplifier is cut off.
A transmitting circuit according to the present invention has a power amplifier and a directional coupler provided between the power amplifier and an antenna, for detecting a reflected wave reflected from the antenna side on the power amplifier side in such a manner that when a level of the reflected wave exceeds a predetermined value, operating current flowing through an amplification element of the power amplifier is reduced.
In the transmitting circuit according to the present invention, the level of the predetermined value is determined based on rated power of the power amplifier and electric power to be transmitted by the power amplifier in such a manner that the power amplifier is not damaged by the reflected wave.
The transmitting circuit according to the present invention has the directional coupler formed of a copper foil pattern on a circuit substrate.
A transmitter-receiver according to the present invention has the above-described transmitting circuit, the transmitting circuit has a duplexer between the power amplifier and the antenna, and the directional coupler is provided between the power amplifier and the duplexer.
A transmitter-receiver according to the present invention has an indicator, and when the level of the reflected wave exceeds the predetermined value, a display screen of the indicator display to such effect.